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Ok, to explain it differently, if the output should be low, the PIC sets the correct bit low and sends it to the 72LS240 74LS240 (ie: the buffer), the 74LS240 will only set the outputs to the CPC when the Com pin coming from the CPC goes low which triggers the output enables of the buffer. So the pin that should be low for exactly 1/300s will be only for 1/300s because that's how long the Com pin went low for. Can that be more exact?